1. Field of the Invention
The present invention relates to a solid-state imaging apparatus, and, more particularly, to a two-dimensional image sensor including a signal processing circuit which reduces noises, and is used for a cellular telephone, a digital still camera, a video camera, a vehicle-mounted camera and the like.
2. Description of the Related Art
A CMOS image sensor which is a kind of two-dimensional image sensor has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-287137 (SUGIKI), Jpn. Pat. Appln. KOKAI Publication No. 2004-15712 (NAKAMURA and KAWAHITO), and Jpn. Pat. Appln. KOKAI Publication No. 2000-261730 (NAKAMURA and SONE). According to one example of the CMOS image sensor, an analog-to-digital conversion (AD conversion) circuit is provided for each of a plurality of vertical lines in an imaging area, and the output of each AD-conversion circuit is sequentially selected to obtain digital video output.
In the CMOS image sensor disclosed in SUGIKI, each AD-conversion circuit has a voltage comparator section. Each of the voltage comparator sections includes an amplifier in a plurality of stages. Furthermore, a clamping circuit is provided in each amplifier. A plurality of pixel signals, which have been read from a plurality of horizontal lines in an imaging area, are converted in the above-described plural of AD-conversion circuits. The data after being subject to the AD conversion is held in each latch circuit, and is output from each latch circuit as data with a plurality of bits in the subsequent horizontal effective period.
In the CMOS image sensor, a number of transistors for switches are inserted on a signal path between the imaging area and the AD-conversion circuit. When threshold voltages of the transistors vary, the levels of the signals fluctuate for each vertical line when the states of the transistors are changed from ON to OFF. Then, noises with a fixed vertical-stripe pattern, that is, vertical-stripe noises are generated by variations in fluctuations of the signal levels.
Moreover, in readout operation for each horizontal line in an imaging area, the power-supply voltage fluctuates, and the waveform of the reference voltage for AD conversion fluctuates for each horizontal line when the states of the above transistors for switches are changed from ON to OFF. Accordingly, the signal levels fluctuate for each horizontal line, and horizontal-stripe noises are generated. The horizontal-stripe noises are generated at random because the noises are caused by fluctuations in the power-supply voltage. These vertical-stripe and horizontal-stripe noises cause deterioration in picture quality.
Furthermore, NAKAMURA and SONE has disclosed that an image signal without vertical-stripe noises can be obtained by a configuration in which a black reference section is provided in the end portion of the effective pixel area in the vertical direction, and, in a vertical blanking period, the output from the black reference section is accumulated to acquire a reference signal by calculating an average value, and to subtract the above-described reference signal from signals in an effective pixel area during each horizontal effective period.